Growth regulation of the mammary epithelium of both normal and neoplastic states is a complex process involving the interaction between the epithelium and stromal cell populations, including the adipocytes. These interactions may be important in the process of preneoplastic to neoplastic conversion of mammary epithelium and the role of dietary lipids in this process. Experiments to date indicate that the epithelium is dependent on essential fatty acids for proliferation and that prolactin stimulated epithelium recruits these fatty acids from mammary adipocytes. Prolactin's role in this process appears to be mediated by signals from the epithelium directed at mast cells in the gland. The activated mast cells release histamine and this compound then triggers the release of fatty acids from the proximal adipocytes. The prolactin activated epithelial cells then selectively take up the unsaturated fatty acids. Part of these are inserted into membrane phospholipids with the consequent stimulation of cell growth. Some of the essential fatty acids are converted to prostaglandins. Of these, prostaglandin E1 is a potent growth stimulator of the epithelium. The essential fatty acids thus appear to be important for mammary cell growth by serving as membrane structural components and as substrates for prostaglandin synthesis.